Forming an uneven part on a surface of a silicon substrate in a solar cell element including a silicon substrate reduces the reflection of light at the light-receiving surface.
A method for forming an uneven part on the surface of a silicon substrate is, for example, a method for using an acid aqueous solution containing at least hydrofluoric acid and nitric acid, and subjecting the surface of a silicon substrate having a damaged layer to etching treatment (refer to, for example, Patent Reference 1).
In this case, the damaged layer on the surface of the silicon substrate is usually formed when a wire saw apparatus is used to slice a silicon ingot thinly to obtain silicon substrates. The wire saw apparatuses used include a free abrasive type and a fixed abrasive type. With the free abrasive type of apparatus, by supplying a cutting fluid containing an abrasive, the silicon ingot is sliced thinly by the lapping action of the wire. With the fixed abrasive type of apparatus, the silicon ingot is thinly sliced by a wire (also called an abrasive-bearing wire) to which an abrasive has been fixed beforehand.
In a solar cell element including a silicon substrate, the silicon substrate includes many metallic elements. The metallic elements are a typical factor in worsening the characteristics of the solar cell element. Transition metallic elements such as iron and copper in particular worsen the output characteristics of a solar cell element. For this reason, the reduction of the worsening of the characteristics of a solar cell element attributed to the existence of metallic elements in the silicon substrate is essential in improving the conversion efficiency of the solar cell element.
Given the above, a technique is necessary in subsequent processes to reduce the influence of metallic elements mixed in the silicon substrate. One known technique is the so-called gettering technique.
Gettering is a technique of moving impurity elements from a region in which the main function of the device is exhibited (also called the device region) to a region other than that region for fixing them. For example, in the case of a solar cell element, by gettering, impurity elements are moved from the device region, which mainly includes an photoactive region and a junction region, to a region other than that region. A typical example of gettering is the gettering of metallic elements by phosphorus diffusion.
For example, a method for manufacturing a solar cell has been disclosed (refer to, for example, Patent Reference 2), wherein, when a pn junction region is formed by thermal diffusion of phosphorus, a layer into which phosphorus has diffused (also called a phosphorus diffusion layer) is used, and the gettering effect acts to improve the characteristics of the solar cell element.